In known apparatus of this type, a source of substantially monochromatic coherent electromagnetic radiation is provided, as is a measuring cell holding the fluid in which the particles are contained. The electromagnetic radiation is split into a measuring beam passing through the measuring cell and the reference beam. The movement of the particles in the fluid under the influence of an applied electric field causes the electromagnetic energy in the measuring beam to be scattered. The radiation scattered at a selected angle to the incoming measuring beam is then heterodyned with the reference beam and the resulting beat frequency is evaluated to determine the Doppler shift of the measuring beam frequency due to the scattering by the moving particles.
When a laser is used as the source of electromagnetic radiation, the above described apparatus allows a relatively rapid and objective measuring of the velocity of particles such as proteins or cells within a fluid. This type of measurement is particularly important in the diagonstic field and is used, for example, to determine the electrophoretic mobility of erythrocytes and leukocytes as well as their sedimentation speed. It can also be used to measure the mobility of sperm.
When the particles in the fluid are weakly scattering particles, the radiation emanating from the glass walls of the measuring cells may be used as the reference beam. For strongly scattering particles, a definite reference beam must be introduced, since the radiation from the glass walls is no longer sufficient.
All the known apparatus of this type has the disadvantage that different measuring angles, i.e., different angles between the incident measuring beam and the selected scattered beam require a new adjustment of the optical components determining the path of the reference beam, so that the scattered and measuring beams will fall on the same location in the mixer where the heterodyning is carried out. Since a series of many measurements must often be carried out in order to obtain information about the type and structure of the scattering particles, a great deal of time is lost in this adjustment of the referenced beam.